Conventional electrically powered lighting systems provided in aircraft for normal use are hard wired with electrical wiring connecting individual light sources to a remote power source, typically storage batteries. Such systems may be rendered inoperable following an accident if the electrical wiring connections to the power source are damaged. For example, the electrical wiring connections may be broken by impact damage to the structure of the aircraft, and/or by fire and/or by water if the aircraft has to make an emergency landing on land or in the sea.
For this reason, it is a mandatory requirement to fit aircraft with emergency lighting systems at ceiling and floor level that are operable independently of the normal lighting system to provide back-up in the event of failure of the latter and to assist evacuation of the aircraft.
We have previously proposed using batteries as a back-up to the aircraft power supply so that, under normal conditions when the emergency lighting is not required, operation of the light units, for example during installation and routine testing of the units, is powered by the aircraft power supply. As a result, the battery power supply is conserved for operating the light units in an emergency when the aircraft power supply fails or the wiring between the aircraft power supply and the light units is damaged or broken such as following a crash.
The provision of batteries as the power source in an emergency can however give rise to issues of reliability due to the life limits of batteries and the present invention seeks to provide an alternative power source for emergency lighting, particularly, but not exclusively, for use in aircraft.